1. Field of the Invention
The present invention relates generally to loading nuclear fuel rods into a fuel assembly skeleton and, more particularly, is concerned with an end cap for a fuel rod gripper of a fuel rod loader which minimizes impacts with grid cell dimples during fuel rod loading operations.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between and connected at opposite ends to the nozzles and a plurality of transverse support grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a multiplicity of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
In assembling a fuel assembly, it has been conventional practice, first, to attach the transverse grids to the longitudinally extending guide thimbles at predetermined axially spaced locations therealong to provide a fuel assembly skeleton. Next, the fuel rods are loaded by passing them through the cells of the grids. Typically, a fuel rod loader is used which pulls the fuel rods into the fuel assembly skeleton, passing them through the grid cells. The loader has a rod gripper which is, first, inserted through the grid cells, next, coupled to the end plug of a fuel rod, and, lastly, withdrawn back through the grids, pulling the attached fuel rod into the grids. After all fuel rods have been loaded by repeating these operations of the gripper, the bottom and top nozzles are then attached to the lower and upper ends of the guide thimbles to complete the fuel assembly.
One fuel rod gripper currently in use is disclosed in U.S. Pat. No. 4,747,997 to D. A. Boatwright. The gripper has a plurality of fingers with end portions which couple with the end plug of a fuel rod. For achieving coupling between the end plug and the gripper, the design of the end plug dictates that the overall configuration of the gripper finger end portions be relatively blunt. Occasionally, when the fuel rod gripper is initially passed through the fuel assembly before coupling with a fuel rod, the leading end portions of the gripper impact a dimple extending into one of the grid cells. The force of the impact due to the blunt configuration of the gripper end portions can be sufficient to cause deformation or cocking of the impacted dimple such that the dimple will not have the desired positional and supporting relationship to the fuel rod once the latter is loaded through the particular grid cell.
Consequently, there is a need for a way to avoid the deleterious effects of impact by the fuel rod gripper with the grid cell dimples when inserting the gripper through the fuel assembly skeleton.